An organic EL device is a light emitting device which emits light when a current passes therethrough as in a light emitting diode (LED), and is also called an organic LED (OLED). For the light emitting display device including a plurality of pixels formed in a matrix form, each of which is comprised of the organic EL device and the drive circuit for driving the organic EL device, an active-matrix (hereinafter, referred to as “AM”)-type organic EL display has been studied.
FIG. 6 illustrates a configuration example of pixels of the AM-type organic EL display. In FIG. 6, reference symbol LED denotes an organic EL device, reference numeral 101 denotes a drive circuit, reference symbol DL denotes a data line, and reference numeral SL denotes a scan line. FIG. 7 illustrates a configuration example of the AM-type organic EL display in which a plurality of pixels is arranged in a matrix form (n columns×m rows). In FIG. 7, reference symbols SL1 to SLm each denote a scan line which is arranged for each row of the first to m-th rows, and reference symbols DL1 to DLn each denote a data line which is arranged for each column of the first to n-th columns. An AM-type organic EL display 100 illustrated in FIG. 7 controls, for each pixel, voltages and currents to be supplied from drive circuits 101 to the organic EL device LEDs through the data lines DL for each column, times, and the like, in response to signals (H level or L level) of the scan lines SL for each row. Through this control, luminance of the organic EL device LED is adjusted, and gradation display thereof is performed.
In the AM-type organic EL display as described above, in a case where the voltage-luminance characteristic of the organic EL device changes over time, the display quality is affected. This also applies to a case where there is variation in characteristics of a thin film transistor (hereinafter, referred to as “TFT”) which is a component of a drive circuit, and a case where the characteristics of the TFT are changed due to an electrical stress to be applied. Accordingly, in order to achieve high-quality display without unevenness, it is necessary to develop a drive circuit and drive method, which is hardly affected by a temporal change of the characteristics of the organic EL device or by the variation and change in characteristics of the TFT.
(Prior Art 1)
FIG. 8 illustrates a simplest drive circuit as a first prior art. In FIG. 8, reference symbol LED denotes an organic EL device, reference numeral 101 denotes a drive circuit, reference symbol DL denotes a data line, reference symbol SL denotes a scan line, reference symbol VS denotes a power supply line, reference symbol GND denotes a ground line, reference symbol D-TFT denotes a driving p-type TFT, and reference symbol C denotes a capacitor. An on/off operation of a switch (switching element) SW1 is controlled in response to the signal of the scan line SL.
In this prior art, the switch SW1 is turned on in response to the signal of a scan line SL, and a voltage from the data line DL is applied to a gate terminal of the TFT (D-TFT), which is provided within the drive circuit 101, through the switch SW1, thereby retaining a voltage between the gate terminal and a source terminal in the capacitor C. The TFT supplies a current to the organic EL device LED according to the voltage applied to the gate terminal. In this prior art, the change in the OLED luminescence is small since the time variation in the current-luminance characteristic of the OLED device is smaller than the voltage-luminance characteristics. On the other hand, when there is variation in characteristics of the TFT, the current supplied to the organic EL device LED varies, whereby display unevenness appears. In the prior art, some drive circuits have been proposed in order to solve the above-mentioned problem. In the following description, prior art examples of those drive circuits will be described.
(Prior Art 2)
FIG. 9 illustrates a drive circuit disclosed in U.S. Pat. No. 6,373,454 as a second prior art. In FIG. 9, reference symbol LED denotes an organic EL device, reference numeral 101 denotes a drive circuit, reference symbol DL denotes a data line, reference symbols SLA and SLB each denote a scan line, reference symbol VS denotes a power supply line, reference symbol GND denotes a ground line, reference symbol D-TFT denotes a driving p-type TFT, and reference symbol C denotes a capacitor. An on/off operation of each of switches (switching elements) SW1, SW2, and SW3 is controlled in response to the signal of the scan lines SL.
In this prior art, the switches SW1 and SW2 are turned on in response to the signal of the scan line SLA, and a current is supplied from the outside (data line DL) through the switch SW1 to the TFT (D-TFT) provided within the drive circuit 101, in which a short circuit between the gate terminal and the drain terminal is formed through the switch SW2. As a result, the voltage at the gate terminal of the TFT can be set as a voltage at which the current flows from the outside according to the threshold and the mobility of the TFT. Then, when the switch SW3 is turned on in response to the signal of the scan line SLB, the TFT serves as a current source and is capable of passing the current having the same intensity as that from the outside to the organic EL device LED through the switch SW3. Accordingly, if the current from the outside does not vary, according to this prior art, it is possible to cause a constant current to flow through the organic EL device and perform display without unevenness irrespective of the characteristic variation of the TFT.
(Prior Art 3)
FIG. 10 illustrates a drive circuit disclosed in U.S. Pat. No. 6,501,466 as a third prior art. In FIG. 10, reference symbol LED denotes an organic EL device, reference numeral 101 denotes a drive circuit, reference symbol DL denotes a data line, reference symbol SL denotes a scan line, reference symbol VS denotes a power supply line, reference symbol GND denotes a ground line, reference symbols L-TFT and D-TFT denote a pair of p-type TFTs forming a current mirror circuit, and reference symbol C denotes a capacitor. An on/off operation of each of switches (switching elements) SW1 and SW2 is controlled in response to the signal of the scan line SL.
According to this prior art, the switches SW1 and SW2 are turned on in response to the signal of the scan line SL, the gate terminal and the drain terminal of one TFT (L-TFT) are short-circuited through the switch SW2, and a current is supplied from the outside (data line DL) through the switch SW1. As a result, the voltage at the gate terminal of the L-TFT can be set as a voltage with which the current flows from the outside. With this configuration, the other TFT (D-TFT) of the prior art TFTs supplies a current to the organic EL device LED according to the voltage. The two TFTs forming the current mirror circuit are positioned closer to each other and there is a small variation in characteristics therebetween, so the current supplied to the organic EL device LED is determined based on the current from the outside and the current capability ratio between the L-TFT and the D-TFT. Accordingly, if the current from the outside does not vary, according to this prior art, it is possible to cause a constant current to flow through the organic EL device and perform display without unevenness irrespective of the variation in characteristics of the TFTs.
For the above-mentioned circuit, a TFT having a channel layer made of a polycrystal-Si (hereinafter, referred to as “p-Si”), amorphous silicon (hereinafter, referred to as “a-Si”), an organic semiconductor (hereinafter, referred to as “OS”), or the like has been studied. The p-Si TFT can be produced with a high mobility at low working voltage, but manufacturing costs therefor are high. On the other hand, the a-Si TFT or the OS TFT can be produced at low cost with a small number of manufacturing steps, but requires a high working voltage and large power consumption because the a-Si and OS the TFT have a lower mobility than the p-Si TFT. Further, a TFT using a metal oxide semiconductor such as ZnO as the channel layer has been developed in recent years, and it is reported that such a TFT has a higher mobility than the a-Si TFT and the OS TFT.
It is difficult for the TFT having the channel layer made of a-Si, an OS, or a metal oxide semiconductor to be of a complementary TFT in which an n-type TFT and a p-type TFT are formed on the same substrate. For example, a p-type semiconductor having a high mobility has not been obtained with a-Si or a metal oxide, so it is difficult to form a p-type TFT. In addition, with regard to the OS, the n-type semiconductor and the p-type semiconductor that have a high mobility are made of different materials, which requires twice as many processes and makes it difficult to manufacture the TFT at low costs. Accordingly, it is necessary that the drive circuit using those TFTs is formed of only the n-type TFT or the p-type TFT.
Further, it is known that the TFT having the channel layer made of a-Si, an OS, or a metal oxide has a current-voltage characteristic which can shift according to the voltage to be applied between the gate terminal and the source terminal.
In the above description, the a-Si TFT is used for a pixel of an AM-type liquid crystal display (hereinafter, referred to as “LCD”) and a production technology therefor with a diagonal size of several ten inches is established. For this reason, the a-Si TFT is regarded as a promising TFT for a drive circuit of a large AM-type organic EL display having a diagonal size of 10 inches or larger, and technology development has been promoted (see fourth prior art as illustrated in FIG. 11 to be described later).
On the other hand, the organic EL device generally has a configuration in which at least a light emitting layer made of an organic material which is sandwiched between an anode electrode and a cathode electrode. The organic material is affected by heat, electromagnetic wave, water, and the like, so characteristics thereof are liable to be changed. For this reason, for a light emitting display device using the organic EL device, it is desirable to use a manufacturing process in which the light emitting layer made of the organic material is formed after formation of the drive circuit and the anode electrode, and then the cathode electrode is formed by vacuum deposition or the like which causes less damage.
According to the above-mentioned process, it is considered a case where each pixel of the AM-type organic EL display includes a drive circuit formed of an n-type TFT, and an organic EL device having an anode electrode, an organic light emitting layer, and a cathode electrode that are formed in the stated order from the bottom. In this case, functions disclosed in U.S. Pat. No. 6,373,454 and U.S. Pat. No. 6,501,466 cannot be achieved only by replacing the p-type TFT with the n-type TFT. This is because, in U.S. Pat. No. 6,373,454 and U.S. Pat. No. 6,501,466, the source terminal voltage of the p-type TFT is fixed by a power supply, and the gate terminal voltage is determined based on the current from the outside. For this reason, at the time of driving the organic EL device, the voltage difference between the gate terminal and the source terminal is fixed, which functions as a constant current source with respect to the organic EL device. In this case, when the p-type TFT is replaced with the n-type TFT, the voltage between the gate terminal and the drain terminal is fixed, which does not function as a constant current source. Further, as described above, the characteristic shift due to the applied voltage is caused, so it is necessary to suppress the influence of the characteristic shift.
(Prior Art 4)
A fourth prior art is a prior art for solving the above-mentioned problem with the drive circuit using an a-Si TFT. FIG. 11 illustrates a drive circuit disclosed in A. Nathan et al. (SID 05 DIGEST, p. 26, FIG. 3) and A. Nathan et al. (SID 06 DIGEST, 46.1, FIG. 1). In FIG. 11, reference symbol LED denotes an organic EL device, reference numeral 101 denotes a drive circuit, reference symbol DL denotes a data line, reference symbol SL denotes a scan line, reference symbol VS denotes a power supply line, reference symbol GND denotes a ground line, reference symbols L-TFT and D-TFT denote a pair of n-type TFTs forming a current mirror circuit, and reference symbol C denotes a capacitor. An on/off operation of each of switches (switching elements) SW1 and SW 2 is controlled in response to the signal of the scan line SL.
In this prior art, the current mirror circuit disclosed in U.S. Pat. No. 6,501,466 is applied. According to this prior art, the switches SW1 and SW2 are turned on in response to the signal of the scan line SL, the gate terminal and the drain terminal of the L-TFT are connected to each other through the switch SW2, and a current is supplied from the outside (data line DL) through the switch SW1. Then, the supplied current flows from the drain terminal of the L-TFT to the source terminal thereof and further to the organic EL device LED. Accordingly, the voltages at the gate terminal and the source terminal of the L-TFT become a voltage with which the current flows from the outside. In addition, the D-TFT has a common gate terminal and source terminal with the L-TFT, so the D-TFT supplies the current to the organic EL device LED according to the gate terminal voltage and the source terminal voltage of the L-TFT. By retaining the gate terminal voltage in the capacitor C, the D-TFT can supply a current which is the same as the current obtained during a period in which the current is supplied from the outside, even in a period in which the current from the outside is stopped.
In addition, during the operation, the gate terminals and the source terminals of the D-TFT and the L-TFT are supplied with the same voltage, and the characteristic shifts of the TFTs become the same. At this time, the current capability ratio between the D-TFT and the L-TFT is retained. In this case, even when the characteristic shift is caused, the current flowing through those TFTs can be made comparable to the current obtained before the characteristic shift is caused.
Note that, in this prior art, it is necessary for the L-TFT to have sufficiently low capability for causing the current to flow, as compared with the D-TFT. This is because the organic EL device is supplied with a current from the L-TFT and the D-TFT during a period in which a current from the outside is supplied, while during a period in which the current from the outside is stopped, the organic EL device is supplied with a current only from the D-TFT. Accordingly, in both periods, the source voltages of the L-TFT and the D-TFT which are determined based on the current capability of the organic EL device do not match with each other when a current value of the L-TFT is larger than that of the D-TFT. In this case, the current set during the period in which the current from the outside is supplied cannot be caused to flow during the period in which the current from the outside is stopped. As a result, it is necessary that the current supplied to the L-TFT from the outside is made smaller than the current supplied by the D-TFT to the organic EL device.
On the other hand, in recent years, the current-luminance characteristic of the organic EL device has been improved, and the current supplied to the organic EL device has been lowered. In addition, there is a demand for a larger and higher-definition organic EL display, and a line load tends to be increased. Therefore, in the prior arts particularly in a case of supplying a low current corresponding to low gradation from the outside, a long time is necessary for charging the line load. In this case, it takes a long time to perform an operation for setting the voltage at the gate terminal of the TFT provided within the drive circuit to be equal to the voltage at which the current from the outside flows, according to the threshold and the mobility of the TFT, which makes it difficult to apply the organic EL device to a display device with high-resolution and a large screen. In order to overcome the difficulty, a unit for increasing the current from the outside may be employed, but the unit cannot be applied to the fourth prior art as described above.